1. Field of the Invention
The present invention relates to steam generators intended primarily to supply natural steam or pressurized steam that is injected into a cooking chamber for foodstuffs.
2. Description of the Prior Art
Steam generators generally consist of a main vessel bounded by a watertight wall that contains water. The water inlet pipework includes a water inlet control solenoid valve in order to inject water into the vessel via at least one water inlet opening. Steam outlet pipework communicates with the upper part of the main vessel via at least one steam outlet opening. Heating devices that can be connected to an external energy source are provided in order to heat the water contained in the main vessel. Control devices control the heating devices and water inlet control solenoid valve in accordance with input signals generated by water level detectors in the main vessel.
In known steam generators, the water level is kept essentially constant in the main vessel. To achieve this, means of detecting the water level are fitted in order to measure the water level and generate control signals for the water inlet solenoid valve.
Various types of methods of detecting the water level are in common use:
according to document EP-A-O 323 939, the water level is detected by a resistive sensor situated in the main vessel which comes into contact with the upper water level; PA1 according to a second embodiment, the heating devices consist of vertical hollow heating tubes that contain the water to be heated and the upper part is equipped with a temperature measuring sensor mounted on the external peripheral surface of the tube; a drop in the water level below the area occupied by the temperature measuring sensor causes a rise in the temperature of the corresponding tube wall, this temperature rise is detected by the temperature sensor and the temperature rise is interpreted as a drop in the water level below the permitted level; PA1 the article entitled LEVEL MONITORING, by Thomas C. Eliott which appeared in POWER No. 9 in September 1990, page 41 ff. states how to measure the water level in a steam generator by means of a pressure gauge but without specifying the means used; PA1 document DE-C-662 932 long ago described how to measure the water level in a tank by using a device with a diaphragm that is in contact with the water. PA1 A measuring vessel, comprising a lower communicating opening, that is associated with a pressure switch that is exposed to the pressure inside the measuring vessel; the pressure switch generates input signals as a function of said pressure and sends them to the means of controlling the injection of water into the steam generator; PA1 Measuring pipework having a first end connected to a measuring opening located in the lower part of the main vessel below the water level and a second end that is connected to said lower communicating opening in the measuring vessel; said lower communicating opening is also located below the level in the main vessel, there is always a volume of air in the measuring vessel between the water and the pressure switch and this air is trapped in the measuring vessel because the normal water level is higher than the lower communicating opening in the measuring vessel. This avoids any contact between the water and the pressure switch. In particular, it prevents the formation of deposited boiler scale or fur on the active components of the pressure switch such as the pressure measuring diaphragm.
These water level measuring devices have proved satisfactory up to now, at least during the initial periods of use. However, these known methods have drawbacks which become apparent either at the start of operation or after a period of prolonged operation.
For instance, detecting the water level by a resistive sensor does not allow correct detection if there is a requirement to produce steam using demineralised water. The resistivity of demineralised water is too high to allow correct operation of a resistive sensor.
In addition, all known methods, regardless of whether they involve a resistive sensor or the detection of temperature or pressure, are particularly sensitive to the presence of deposits of boiler scale or calcareous fur which inevitably form after the steam generator has been used for a more or less prolonged period. Deposited boiler scale or fur on a resistive sensor significantly alters the electrical signals produced by the sensor. Similarly, deposited boiler scale or fur on the internal wall of a heating tube in the area occupied by the temperature sensor significantly alters the operation of said sensor because the film of boiler scale or fur acts as a thermal insulator. Deposited boiler scale on the diaphragm of a pressure gauge significantly alters the ability of the diaphragm to change shape and affects detection accuracy.